PSI - Issue 42

Lewis Milne et al. / Procedia Structural Integrity 42 (2022) 623–630

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Lewis Milne et al. / Structural Integrity Procedia 00 (2019) 000 – 000

3.3. Test Procedure

The high-frequency testing was carried out at 20 kHz using a Shimadzu USF-2000A ultrasonic testing machine. Threads were used to attach the specimen to acoustic horns on both sides and the height of the top horn was adjusted to achieve zero mean stress. The automatic fracture detection feature was enabled, which stops the test when a crack is formed by monitoring the change in the resonance frequency. To cool the specimen, forced air convection was applied using cooling nozzles blowing compressed dried air over the sample, and intermittent load pulses were used. The loading pulse was kept at 110 ms, and the cooling pause was adjusted between 0.5-5 s depending on the rate of heat generation. This allowed the test duration to be minimised by using shorter cooling pauses near the beginning of the test when there was little heat generation, while utilising longer cooling pauses towards the end when the heat generation was greater. Specimens were painted with a high emissivity black paint, and an infrared (IR) spot sensor with a sampling interval of 20 ms was used to monitor the specimen temperature. To ensure the test stayed at room temperature, the IR sensor was fed into a control system which would stop the test should the specimen exceed 30 °C. The cooling pause would then be increased and the test started again. Photos of the test setup are shown in Figure 2 (a) and (b). The low-frequency testing was carried out at 20 Hz using an Instron 8801 servohydraulic testing machine. The testing was carried out in environmental conditions and at room temperature, to keep all environmental influences consistent between the two tests. A photo of the test setup is shown in Figure 2 (c). 4. Results and discussion 4.1. Fatigue Results The SN curves produced for Q355B at 20 Hz and 20 kHz are presented in Figure 3. The SN curves produced at low frequency show a good correlation with the traditional power law equation, however no clearly defined fatigue limit can be observed, as one specimen ran out at 300 MPa and another specimen failed at 270 MPa. The UFT curve exhibits a large amount of scatter in the data, with an R 2 value of 0.4264 when fitting to a power law curve. Additionally, all of the tested samples failed and so a fatigue limit value cannot be determined. This large amount of scatter in the fatigue results is to be expected b ased on the manufacturer’s experience .

500

450

400

350

Q355B 20Hz Q355B 20kHz Run Outs

300

250

Stress Amplitude (MPa)

200

150

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

Number of cycles to failure (cycles)

Figure 3: SN curves at 20 Hz and 20 kHz for Q355